In wireless transmission apparatuses in a wireless communication system, an amplifier to amplify power of a transmission signal is equipped. In the wireless transmission apparatus, generally, an amplifier is operated near the saturation region of the amplifier to enhance power efficiency of the amplifier. However, when an amplifier is operated near the saturation region, nonlinear distortion increases. Therefore, to reduce adjacent channel leakage power (adjacent power leakage ratio (ACLR)) by suppressing this nonlinear distortion, a distortion compensation apparatus that compensates nonlinear distortion is equipped in the wireless communication apparatus.
As one of distortion compensation schemes used in a distortion compensation apparatus, there is a “pre-distortion (hereinafter, referred to as “PD” in some cases) scheme”. The distortion compensation apparatus applying the PD scheme suppresses distortion in output of an amplifier by enhancing linearity of output of the amplifier by multiplying a transmission baseband signal before input to the amplifier by a distortion compensation coefficient that has inverse characteristics of nonlinear distortion of the amplifier. A signal obtained by multiplying the transmission baseband signal by the distortion compensation coefficient is referred to as a “pre-distortion signal (PD signal)” in some cases. Therefore, the PD signal is a signal distorted in advance according to the inverse characteristics of the nonlinear distortion of the amplifier before being input to the amplifier.
For example, as the distortion compensation apparatus of the PD scheme, one that has a lookup table (hereinafter, “LUT” in some cases) in which distortion compensation coefficients are stored, and that reads a distortion compensation coefficient from the LUT, specifying an address corresponding to power of a transmission baseband signal to the LUT is available. The distortion compensation coefficient stored in the LUT is sequentially updated such that an error between a signal that is obtained by comparing a transmission baseband signal as a reference signal and a signal that is output from the amplifier and fed back thereto (hereinafter, “feedback signal” in some cases”) is minimized. Conventional examples are described in International Publication Pamphlet No. WO 2006/033256 and Japanese Laid-open Patent Publication No. 2007-208684.
At the comparison between a transmission baseband signal as a reference signal and a feedback signal, adjustment processing to match timing and phase of the both signals is performed. Because transmission time in the apparatus can be regarded as unchangeable to be constant, timing adjustment processing to match the timing of the both signals may be performed, for example, before sell-in. On the other hand, the phase characteristics of electronic parts (for example, an amplifier) in the apparatus vary according to ON/OFF of the apparatus or with time, and therefore, phase adjustment processing of the both signals is performed each time.
However, noise components can be superimposed on a feedback signal (that is, an output signal of the amplifier being a transmission signal), and when power of the feedback signal is small, the feedback signal can be buried in the noise components. In this case, the phase-specific accuracy using the feedback signal is deteriorated, and the accuracy of the phase adjustment processing can be deteriorated. As a result, the accuracy of the distortion compensation processing can be deteriorated.